Describes the development and validation of the Developmental Behavior Checklist (DBC), a standardized instrument completed by lay informants to assess behavioral and emotional disturbance in children and adolescents with mental retardation (MR). Items describing common behavioral and emotional problems in this population were generated by extracting descriptions from 664 case files of children and adolescents with behavior disorders seen at a specialist developmental assessment service over 12 years. These items were reduced to a set of 96 items administered to a sample of 1,093 children and adolescents with mental retardation and then submitted to a principal components analysis. Six interpretable and partly validated subscales were obtained which explained 36% of the total variance and had satisfactory internal consistency. Interrater and test-retest agreement were satisfactory for both total scale score and for scores on each of the subscales. Good evidence of concurrent validity was provided by substantial positive correlations between total scores on the DBC completed by lay informants and the ratings of experienced psychiatrists based upon interviews and scores on two standardized instruments that must be completed by health professionals. The discriminative validity of the total score as assessed by area under the ROC curve was excellent (92%). Standardized norms for the DBC are derived from an epidemiological study of behavior problems in children and adolescents with mental retardation undertaken in two Australian States. Norms are available for the mild, moderate, severe, and profound MR groups and for the MR population as a whole.